The present invention relates to management and control of mobile telephone communications and, in particular, to use of information representing one or more of the location, speed and direction of travel of mobile stations to reduce the instances of call disruption and enhance the quality of service.
Mobile telephone systems rely on a variety of management and control techniques to provide simultaneous services to large numbers of mobile stations, even though the spectrum of radio frequencies available for this purpose is quite limited in a relative sense. Each of the multitudes of mobile telephones in use at any given moment in a mobile system must be allowed to communicate with the stationary, ground-based network separately from others. However, insufficient room exists across the spectrum of radio frequencies available for all users to utilize a separate radio frequency to conduct a conversation or data transmission.
The most basic management and control technique used universally by mobile systems is to provide an array of adjacent xe2x80x9ccellsxe2x80x9d that separate the use of any given radio frequency by distance. While the cells cover the service area like patches in a quilt, each cell employs a set of radio frequencies or channels to communicate with mobile stations within the cell that are different from the set of frequencies used by immediately neighboring or adjacent cells. In this way, each cell contributes to separation by distance of frequency sets of other cells around it. This separation therefore allows frequencies to be reused throughout the array of cells, generally without overlap or interference. Separate calls can be made by different users over the same frequency in different cells spaced sufficiently apart. Each cell may also be further subdivided into sectors, each using separate sets of frequencies or channels. Three sectors are typically used in a cell.
The number of mobile users serviced and the capacity of the system are increased further by dividing each frequency used within a cell or sector into a number of separate time slots. Each mobile phone operating on that frequency or frequency band is assigned a separate time slot in which to broadcast transmissions to the base transceiver station (BTS) within its cell. The BTS broadcasts to the mobile station in a similar manner, sometimes on the same and sometimes on a different frequency. Thus, a number of mobile stations can use the same frequency to communicate with the same BTS within the cell, separately and without interference. This sort of division of frequencies into time slots or intervals is widely known as Time Division Multiple Access (TDMA).
Other techniques for providing access to the mobile system by multiple users within the same cell are Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). Each technique is well-known by persons who are skilled in the field of mobile telecommunication systems and therefore need not be discussed in more detail.
Maintaining substantially continuous communication with a mobile station as it moves from one cell or sector to another and a high quality of service (QOS) are objectives to which existing systems aspire. Achieving these objectives would result in service similar to that provided by land-line communication networks that do not rely on radio or other electromagnetic transmission. Meeting these objectives, however, in FDMA and TDMA systems is hampered by the need to change the frequency or channel through which a mobile station communicates as it travels into an adjacent cell or sector within a cell. In general, the terms xe2x80x9ccellxe2x80x9d and xe2x80x9csectorxe2x80x9d are often used interchangeably; however, their meanings differ in the sense that cells are formed by different BTSs, while sectors share the same BTS and together form a larger cell. Because the adjacent cell or sector operates using different sets of frequencies to avoid interference, the mobile station must adjust to the frequency environment of the cell or sector into which it travels. This sort of adjustment is known as a xe2x80x9chard hand-off.xe2x80x9d
In TDMA systems, this inter-cell (adjacent cells) or intra-cell (adjacent sectors) xe2x80x9chand-offxe2x80x9d of the traveling mobile station to the adjacent cell or sector is made more difficult by the need to synchronize the time slot in which the mobile station transmits and receives xe2x80x9cburstsxe2x80x9d of signals to and from the associated BTS. This time slot adjustment is required to avoid interfering with the transmission bursts of other mobile stations operating over the same frequency. In addition to resetting the time slot assignment, when a mobile station travels from a servicing cell to a target cell having radii appreciably different, the transmission of the mobile station must be retarded or advanced to compensate for a different transmission or xe2x80x9cpropagationxe2x80x9d delay in transmissions to the BTS of the target cell. This burst transmission adjustment compensates for any appreciable difference in the transmission propagation delay resulting from different lengths of the transmission paths to the BTS of the prior serving cell and the new target cell. Under existing standards, such burst transmission adjustments are necessary for approximately every 500 meter difference in transmission distance; however, circumstances may make adjustments based on other increments desirable.
Because existing TDMA systems currently do not determine this distance in advance of the hand-off, transmission to and from the mobile station must be temporarily interrupted or xe2x80x9cmutedxe2x80x9d while the mobile station transmits small bursts of synchronization codes to the BTS of the target cell, in accordance with IS-54B and IS-13 standards. The BTS determines the correct burst advance or retard to be implemented and transmits the information to the mobile station, which completes the adjustment. This muting avoids undesirable burst collisions with other mobile stations operating on the same channel or frequency while the adjustment is made. Unfortunately, an undesirable consequence is that the mobile station user is temporarily disconnected from any communication and typically hears a xe2x80x9cclickxe2x80x9d noise. Therefore, it would be desirable to provide a management and control system that adjusts the burst transmission of the mobile station in connection with the hand-off with less disruption of communication.
Other difficulties experienced with current TDMA management and control systems during hand-off stem from system determinations of what new frequency of the target cell or sector to assign a mobile station. Mobile stations operating with systems employing the Mobile Assisted Hand-off (MAHO)procedure of the IS-136 standard frequently monitor the strength of the Digital Control Channels (DCCHs) of immediately adjacent cells (as distinguished from the traffic or voice channel or frequency) and provide this information to the BTS of the currently serving cell. Using this information, the BTS communicates to the mobile station its preferred DCCH assignment and adjustment in the target cell.
However, mobile stations sometimes incorrectly monitor a stronger DCCH signal of a cell that is not immediately adjacent, but that is of the same frequency as a DCCH of an adjacent cell. This often occurs due to shielding of the adjacent cell DCCH by buildings in urban areas, for example. Such a xe2x80x9cfalsexe2x80x9d readings will result in a dropped call as the hand-off is made to the incorrect, unavailable or unacceptably weak DCCH of the actual target cell. It is therefore desirable to provide an ability to distinguish or eliminate such xe2x80x9cfalsexe2x80x9d readings.
When a target cell or cell sector does not have available a frequency or channel for one or more mobile stations moving into that cell or sector, each station is placed by the management and control system in a xe2x80x9cqueue,xe2x80x9d while awaiting availability of a channel. Placement of a mobile station in the xe2x80x9cqueuexe2x80x9d by the system for such inter- or intra-cell hand-off and channel assignment, is triggered primarily by a reduction of signal strength of the BTS to a predetermined level.
However, such reliance on signal strength sometimes undesirably causes the broadcast of the cell serving the mobile station to be xe2x80x9cdraggedxe2x80x9d into the target cell and perhaps other adjacent cells during inter-cell hand-offs, particularly when the mobile station is traveling quickly relative to other mobile stations in queue. This is caused by the serving cell BTS increasing the power level of its transmission to maintain contact with the mobile station. The resulting intrusion of dragged transmission into the adjacent cells violates principals of frequency reuse and separation, and also causes other problems, such as false readings of a non-adjacent DCCH. In intra-cell, sector-to-sector hand-offs, the strength of signals broadcast by the directional beam antennae of a sector attenuates more rapidly as the sector boundary is approached. It is therefore desirable to provide an ability to hand-off channel assignments more rapidly to those mobile stations having a greater need for such an assignment.
As a mobile station travels within a cell or sector, differences in local topography, buildings, fading effects, noise sources and the like often cause attenuation of or interference with signals reaching the station, due to shadows, reflections, multipath and the like. Undesirable degradation of the Quality of Service (QOS) results. Current systems typically compensate for these signal losses by increasing the power level of the channel, in response to frequent monitoring of the channel power level received by the mobile station and feedback by the mobile station to the BTS. However, this power level adjustment is usually completed only after the QOS reduces, to an extent noticeable to the mobile station user. It is therefore desirable to compensate for, reduce or substantially eliminate QOS degradation caused by such geographical differences.
The foregoing difficulties and needs are addressed by the present invention, which enhances wireless systems management, control and quality of service. Information representing the geographic position of a mobile station is provided to the system for the purpose of advancing or retarding transmissions to the BTS of a target cell or sector during a hand-off. Use of this information reduces or avoids the muting period currently needed to allow synchronization of the mobile station transmissions to the target cell or sector.
In another aspect, information representing the position of the mobile station is used to distinguish transmissions of adjacent cells from non-adjacent cells.
This reduces or substantially avoids false readings of channels of non-adjacent cells in a hand-off procedure.
In still another aspect, information representing one or more parameters of the position, speed and direction of travel are used to adjust the priority of target cell channel assignment amongst two or more mobile stations requiring a hand-off to a target cell or sector.
This more rapidly assigns new channels to those mobile stations having a more urgent need and reduces the number of dropped calls during hand-off.
In yet another aspect, information representing the position of a mobile station is used to anticipate or predict the power level requirements at two or more, and preferably many, locations within a cell or sector. The position information can also be used to reassign the mobile station to a channel that is preferable for use in the location of the mobile station. These adjustments and assignments can be made with reference to quality of service data obtained and correlated to position information as the mobile station travels during active calls.